November. 1931
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Experimental Evidence in Support of Balanced-Layer Theory of Liquid !'F Formation' C. W. Foulk and J. N. Miller DEPARTMENT OF CHEMISTRY, Orno STATE UNIVERSITY, COLUMBUS, OHIO b
In this article are given: a restatement of the balanced-layer theory of liquid film formation; a description of two new forms of apparatus for studying film formation; a discussion, with data, on t h e relation of the static and dynamic surface tensions of a solution t o the sign and the magnitude of t h e surface adsorption; various tables giving data t o show the interdependence of the difference between static and dynamic surface tension, foaminess, and tendency towards film formation of solutions.
servation to be made, it can be called a film. In the case of pure liquids there is no resistance to the approach of the two surfaces, and therefore no film is formed.
HIS paper contains a r e s t a t e m e n t of the balanced-layer theory of liquid film formation, as a d v a n c e d by one of the authors (1) in 1929, and a record of c e r t a i n e x p e r i Resistance of Thin Surface mental evidence in support Areas of that theorv. It is Dart The source of the resistance of an i n v e s t i g a t i o n o i t h e to the mechanical force pushfoaming and priming of boiler water, the first paper ( 2 ) of which appeared in 1924. The ing two liquid surfaces together lies in the difference in conearly stages of the work were greatly hampered try the lack centration between the surface layer and the mass of a of a general theory of film formation. This was so keenly solution. Solute is either more (or less) concentrated in the felt that efforts were directed towards finding such a theory, surface than in the mass; that is, solute is either positively and in 1929 a paper was published along this line in the or negatively adsorbed in the surface. This movement of Second Progress Report of the Boiler Feedwater Studies solute into or away from the surface takes place spontaneously against diffusion, and it will therefore require work Committee (3) and subsequently in more detail in INDUSTRIAL done on the system to restore the equality of concentration. AND ENGINEERING CHEMISTRY (1). Very little experimental evidence was offered in these papers, but promise was made In other words, thin surface layers of solution resist an atof the publication of experimental work then in progress in the tempt to mix them. senior author's laboratory. This promise is now fulfilled. I n Figure 1 an attempt is made to depict the two cases of surThe second paper just referred to contains a full discus- face-bubble formation. d is a solution exhibiting negative adsion of this theory of film formation, together with the au- sorption of dissolved matter in the surface, and B is one showing thors' obligations to previous workers, because much that is in positive adsorption. The light hatching around the interior of the theory had already been stated, but as applying to special bubble 1 represents, on an exaggerated scale, the interior thin of less concentration than that of the mass of the liquid. At cases. The theory will now be restated and tht: promised layer 2 this bubble is represented as having risen to the top and having experimental evidence offered. pushed up a portion of the surface layer, also less concentrated
T
P A R T I-THE
BALANCED-LAYER T H E O R Y O F F I L M FORMATION
Formation of Films Liquid films are always formed by the approach t o each other, usually with an extension of area, of two already formed liquid surfaces. The proof of this point of view lies in the fact that, by starting with a mass of liquid, only two general ways of forming thin layers can be imagined-one by the method outlined in the foregoing, and one by the creation of a new surface, as, for example, by tearing apart a column of liquid. These two general methods-the utilization of two already formed surfaces or the creation of a t least one new surfaceare mutually exclusive, and since the creation of a new surface requires the expenditure of several million times as much energy as the extension in area of an already formed surface, the likelihood of surfaces being formed in that way is very slight. In the case of solutions, the mechanical force acting to bring the surfaces together meets with resistance when the layer of liquid between the surfaces becomes very thin. The movement of the surfaces towards each other therefore ceases, when the mechanical force pushing the surfaces together is just balanced by the resistance which it encounters. A state of equilibrium is reached and, if the thin layer of liquid persists long enough (is sufficiently stable) for an ob1 Received July 24, 1931. Presented in substance before the Ninth Colloid Symposium at Ohio State University, Columbus, Ohio, June 11 to 13, 1931.
than the bulk of the solution. Between these two surface layers is a thin layer of the solution of the bulk concentration. The two surface layers are pictured a t 2 as having ceased their approach t o each other because any further approach would result, in effect, in mixing them; that is, i t would cause uniform concentration of the three layers, the two surfaces, and the interior one of bulk concentration. This tendency t o mix is resisted by the physico-chemical forces which brought about the differences in concentration, and consequently a film is formed. Exactly the same set of conditions is pictured in B , except that positive adsorption is exhibited a t 11; at 21 there are again two surface layers differing in concentration from the bulk of the solution, and between these two layers is one of the bulk concentration.
This theory harmonizes the puzzling fact that both positively and negatively adsorbed dissolved matter causes foaming, that is, film formation, and explains why pure liquids do not foam. Corollaries Certain corollaries follow naturally from the theory. (A) I n the case of solutions containing both positively and negatively adsorbed substances, there should be certain mixtures which do not foam because positive and negative adsorption cancel each other and thus produce equality in concentration between surface layer and mass. The same idea can be expressed by saying that the foaming of solutions of positively adsorbed substances will be stopped by the addition of the right amount of a negatively adsorbed material, and vice versa. This provides a theory for one type of antifoam. (B) In the case of those solutions which contain only one
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solute, but which either do not foam or foam a t certain concentrations only, it will be found that the condition of nonfoaming is alaQ. 8 condition of equality of concentration betweeq d c e , and mass of the solution. Finally, it should be noted that the theory is silent on the question of&bility of films. Stability is a relative term and largely a function of the conditions to which the film is subjected after its formation. PART I1 -EXPERIMENTAL TECHNIC
Before presentin'g their experimental evidence, the authors wish to anticipate certain criticisms. It will be seen that the experiments are not exhaustive. They could have been extended over a much greater variety of substances. The quantitative r e s u 1t s -7 are not very Drecise. and finally &ere are no molalities or activities and no therm o d y n a m i c s . The limitations of time are offered as an excuse for a part of t h e s e omissions, and for the n D r est, a hesitation Figure 1-Sketch of Negative and Positive Adsorption and Resulting Balanced a b 0 u t r u s h i n g in Layers in Bubble Films on Surface where theoretical colloid c h e m i s t s have feared to tread. In brief, what is offered here is only a suggestion of certain new points of attack on this difficult problem of the foaming of boiler water. The arrangement followed in presenting the experimental work is, first, to give the three lines of experimentation that were used and then, in Part 111, to offer the results in tables and graphs so that the three sets of data can easily be compared. Dynamic Foam Meter
The apparatus illustrated in Figure 2 was designed by Hansley (4) while working on the foaming of such salt solutions as occur in steam boilers. It is nothing more than an arrangement for blowing air through a porous septum into a column of liquid. The air was filtered through cotton wool and passed through soda lime to remove carbon dioxide. Constant pressure throughout the experiments was also maintained. If a pure liquid or a very dilute solution is in the tube, the small bubbles emerging from the pores of the septum merge into larger ones on being crowded against each other, and these into still larger ones. The tube becomes full of large bubbles, and there is little or no foam on the surface. If a highly purified liquid were used and its own vapor bubbled through in order to eliminate the small amount of air dissolved, there would probably be no trace of foam on the surface. Now if some solute is added in gradually increasing concentration, it will be seen that the bubbles become gradually smaller; that is, the coalescence of small bubbles into larger ones becomes gradually less because of the resistance to mixing offered by the layers of solution constituting the interior surfaces of the bubbles, until a t a certain concentration such coalescence nearly ceases, and the liquid takes on a white appearance due to the mass of tiny air bubbles in it. I n other words, the bubbles remain the size they have on emerging from the pores of the septum. It will further be observed that a layer of foam forms on the surface of the solution and grows thicker as the concentration of the salt increases.
The instrument has been called a dynamic foam meter because the observations are made while the formation of bubbles and foam is going on. The thickness of the layer of foam is determined by the steady state between this rate of formation of the bubbles and the stability of the films. This stability may be very low and still permit a measurement of the foam layer, thus making the apparatus suitable for measuring the foaminess of those solutions which give very unstable foams-boiler water, for example. So far as the formation of films is concerned, the apparatus shows admirably that small bubbles merge together in pure liquids t o form larger ones; that is, no permanent liquid film is formed between the interior surfaces of two approaching bubbles. If, however, something is dissolved in the liquid, there is a resistance to a coalescence of the bubbles; that is, the thin layer of liquid between two approaching bubbles does not burst and the tubbles roll past each other without merging. The gradual diminution in the size of the bubbles, as the concentration of the solute increases, shows also that the tendency to film formation increases with the concentration, a t least up to a certain value. This point has not been specially studied. The difference in curvature between large and small bubbles may play a role here in the sense that two very small bubbles may have a greater tendency to merge than two large ones, and since the pressure inside a small bubble is greater than that inside a large one, a small bubble, on coming close to a large one, may burst into it. It will be shown later under the two-bubble e x p e r i m e n t that the merging of large bubbles also decreases with increasing concentration. Corollary (A) is easily tested in this foam meter. It is necessary only to add enough of a substance to produce a marked effect on the size of bubbles and then to add cautiously a substance w i t h the o p p o s i t e surface-adsorption sign. For example, the foaming of a solution of the n e g a t i v e l y a d s o r b e d sodium sulfate is stopped by the addition of a little soap which is positively adsorbed. There are certain points in the behavior of solutions in this foam meter that relate particularly to foaminess and to film stability. When the air pressure has been properly adjusted (this is of course determined by the porosity of the particular septum in use) , it will be seen that two sharply defined zones are produced. The upper zone, F , consists of true foam, and the lower zone, B, of a column of liquid through which the air bubbles are rising. It can be called the bubble zone. It is evident that in a given case the height of the foam zone is determined by the length of time a foam bubble lasts after it leaves the bubble zone, and by the rate a t which it Figure 2 - D y n a m i c r i s e s t h r o u g h the foam zone. The Foam Meter height of the yoam zone is proportional then to the stability of the foam bubbles or films &nd t o the rate a t which the foam bubbles are moving up the tube. This of course does not mean that the height of the foam zone may not be affected by other conditions-for example, the size of the foam bubbles, It is a fair guess that, in testing a series of solutions, if the same initial volume and
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the same air pressure are used in each case, the f o a q stabilities will be proportional to the heights of the foam zones. The outcome of such a series of tests on molar solutions of various salts is shown in Table I. Table I-Effect of Nature of Salt on Foaming of Molar Solutions HEIGHT OF FOAM SALT^ COLUMN Cm. 4.8 4.9 5.0 5.2 5.6 6.5 7.6 9.3
NaCNS K C_ NS _. _
5
CaCIz KCI KaC1 NazCOa NazSOi MeSOi All of the salts are negatively adsorbed except NaCKS and KCXS.
An inspection of Table I suggests that the salts in the lower end of the column give the more stable foams. rnThe salts whose behavior is presented in Table I1 were selected as typical of different types with respect to film f o r m a t i o n and foam stabilitv. Sodium ch1oride”and sodium sulfate are negatively adsorbed and sodium sulfocyanate is Dositivelvadsorbed. Figure 3-Apparatus for Bringing Two It will be Seen that Bubbles Together under Surface of Liquid the foam height increases rapidly a t first with the concentration, then exhibits a tendedcy towards constancy. The authors suggest that this, together with the results in Table I, indicates that the stability of films of such salt solutions tends toward a constant value under a given set of conditions.
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On touching each other they will either coalesce to form one larger bubble, or they will not coalesce but will roll past or flatten against each other. If they do not coalesce when pushed together, it means that a liquid film is formed between them. In Figure 4 an attempt is made to show the successive appearance and behavior of the bubbles when they fail to coalesce. I n I the production of the bubble in the lower tube A of Figure 2 is shown. I n I1 the bubble is caught in the bell; in I11 a second bubble is emerging from the lower tube; and in I V it is seen pressed against the upper one with the film between the two bubbles. The appearance of the breakaway is shown in V and VI.
The two-bubble experiment is thus seen to be a simple method of studying film formation. It should be pointed out further that the conditions of such an experiment are most favorable for a study of film stability. A film produced under the surface of a liquid is protected from evaporation and from rupture due to the running out of liquid from between its surfaces. It needs only to withstand mechanical shock and a too great force pushing its surfaces together. The technic of the experimental work was as follows:
lu*), of Concentration on Foaming of Solutions HEIGHT OF FOAM ZONE 0 . 2 M 0.4 M 0.6 M 0 . 8 M 1M Cm. Cm. Cm. Cm. Cm. Cm. 5.1 5.1 5.4 5.6 1.9 2.8 6.1 6.5 7.6 1.6 3.2 5.4 2.3 1.2 1.2 3.9 4.8 1.1
Table 11-Effect SALT NaCl h’arSO4 NaCNS
0.05 M
First, with a given set of conditions that could be kept fairly constant, the average time of a thousand contacts was found. Counts were then made of one of the two possible behaviorscoalescence or non-coalescence-of the bubbles over this time interval and the results recorded as percentage of film formation, that is, the number of times in a hundred that the bubbles failed to coalesce. The term “film formation” is used instead of the expression “failure to coalesce.” For example, on observing the hundred contacts of a hundred pairs of bubbles in a given solution, if it was noted that 10 of the pairs failed to coalesce on touching each other, the result was stated as 10 per cent film formation under the conditions. This is logical, because a failure of two bubbles to coalesce into one is due to the formation of a film between them.
A a“ C YQ n
n
Talmud and Suchowolskaja (11)recently published a paper in which they show, by determining the number of seconds during which individual bubbles lasted on the surface of various inorganic salt solutions, that the stability of‘ such bubbles increased rapidly with the concentration, and in most, but not all, cases quickly reached a constant value. Behavior of Two Bubbles in C o n t a c t i n a Solution
If it were possible to segregate two, and only two, bubbles from the hundreds in the foam-meter tube and then herd them towards each other in a salt solution, with some control over the force with which they are pushed together, it is obvious that their behavior could be studied in greater detail. Such a procedure is possible and has come to be known in the Ohio Laboratory as the two-bubble experiment. The apparatus employed is illustrated in Figure 3. It consists of a bent tube, A , opening under a small glass bell, B, so placed that a bubble issuing from the tube will be caught and held in such a way t h a t a second issuing bubble will touch it. Another form of the apparatus has also been used, in which two bubbles issuing simultaneously from the ends of two tubes in a solution touch each other. A regulating device is shown a t C. The peculiar shape of the openings in the capillary tubes was found best for the production of the bubbles.
It is clear that in the operation of this apparatus, the bubbles can exhibit only one of two possible behaviors:
Figure 4-Sketch of Successive Steps of Two-Bubble Experiment
The behavior of the bubbles in this experiment is in entire harmony with the theory. I n water or other pure liquids the bubbles coalesce at once on touching each other, thus giving a direct proof that under these conditions two approaching liquid surfaces continue the movement towards each other till the intervening layer of liquid breaks. If some salt is now added in gradually increasing concentration, it is found that the percentage of film formation (failures to coalesce) also gradually inc\reases till it is nearly 100 per cent. Furthermore, film formation is promoted by both positively and negatively adsorbed substances. Table I11 gives a picture of the results obtained. An inspection of Table I11 shows that film formation under the experimental conditions begins, in the case of sodium sulfate, a t 0.03 molar or less, and that it rises rapidly a t first with increasing concentration.
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Table 111-Nature and Reproducibility of Results Obtained i n TwoBubble Experiment FILM FILM CONCN. FORMATION CONCN. FORMATION
%
70
SODIUM SULFATE
5000 p. p. m. 0.035 M
1.7 1.8 1.7 2.1 1.5 Av. 1 . 8
SAPONIN
10 p. p. m
48.6 50.1 49.3 49.5 49.6 Av. 4 9 . 4
the static and dynamic surface tensions of the solutions had any relation to the results with the foam meter and the two-bubble experiments described previously. Papers on vibrating jets by Schmidt (9) and Stocker (10) were followed in determining the dynamic surface tension. Static surface tension was measured by the drop-weight method, the calculations being made according to Iredale. (6),with the use of appropriate data obtained by Harkins (5). The salts used in preparing solutions for the surface tension measurements were not specially purified but were of good analytical grade. The distilled water employed was redistilled and preserved in Pyrex flasks. PART 111-COMPARATIVE RESULTS
ORDINARY DISTILLED WATER
0.1 0.0 0.0 0.3 0.5 Av. 0 . 2
Dynamic vs. Static Surface-Tension Measurements
It is well known that all solutions do not foam, that is, do not form films. Strong solutions of cane sugar, for example, though very viscous, do not foam, and solutions of sulfuric acid fail to foam at certain concentrations. I n corollary (B) it has been stated that such non-foaming solutions will be found to exhibit no difference in concentration between surface layer and mass. It is a rather difficult task to prove this, however, even in view of the recent work on surface concentration by McBain and his students (7, 8 ) . It was thought, nevertheless, that something could be learned by an indirect attack, and, accordingly, attention was turned to dynamic and static surface-tension experiments. As the terms are used here, dynamic surface tension means a measurement made on a surface within a small fraction of a second after the formation (extension) of the surface; that is, before there has been time for the difference in concentration between surface layer and bulk of the solution to establish itself. Such measurements are made by determining the distance between the nodal points on a stream flowing from an elongated orifice. Since such a stream is moving at a rapid rate, and since its surface dates from its emergence from the orifice, the measurements are made within a small fraction of a second after the formation of the surface. Static surface tension, on the other hand, is that of an old surface-one in which there has been time for the difference in concentration between it and the bulk of the solution to establish itself. I n the case of pure liquids the two measurements are the same, because there is no difference in concentration between surface and mass. I n the vast majority of solutions, however, there is such a difference in concentration and it is upon this difference, among other factors, that the value of the static surface tension depends. It takes time for this difference in concentration to establish itself, and therefore the surface tension of a freshly formed surface-one that has been in existence for only a fraction of a second-will be different from that of an old surface. It may reasonably be supposed that ih those solutions in which there is a large difference between dynamic and static surface tension, there is also a large difference between surface and mass concentration; conversely, in the case of those solutions in which there is no difference between dynamic and static surface tensions, there is also no difference between surface and mass concentrations. MEASUREMENT OF SURFACE TENSIONS-NO details of the methods employed are given because no attempt a t any originality or special accuracy was made. The sole object of the measurements was to see whether the differences between
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An inspection of Table IV shows that the static and dynamic surface-tension differences, the foam-zone heights, and the percentage of film formation all have the same trend. The results are represented graphically in Figure 5. Table IV-Comparison of Differences between Static and Dynamic Surface Tensions, Foam-Zone Heights. and Percentage Film Formation +URFACE TENSIONFOAM-ZONE FILM SALT' Static Dynamic Di5erence HEIGHT FORMATION Dynes/cm. Dynes/cm. Dynes/cm. Cm. % 71.0
71.5 75.0 72 3
0.5 0.9
4.8 5.0
1 3
5.2
29 31 44
.-..
0
72.6 2.3 Molar concentration. NaCKS is positively adsorbed.
The results in Table TT show again the corresponding trends of static and dynamic surface-tension differences, foam-zone heights, and percentage film formation. Table V-Effect of Concentration upon Difference between Static and Dynamic Surface Tension, Foam-Zone Height, and Percentage Film Formation MOLAR -SURFACE TENSION-FOAM-ZONE FILM CONC. Static Dynamic Difference HEIGHT FORMATION Dyneslcm. Dyneslcm. Cm. % SODIUM SULFATE
1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.05
74.9 74.6 74.7 74.5 73.9 73.6 73.5 73.2 73.0 72.6 72.3
1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1
73.8 73.6 73.3 13.3 12.9 72.7 72.5 72.5 72.4 72.3 72.2
72.6 72.6 72.4 72.5 72.5 72.3 72.4 72.1 72.3 72.2 72.2
2.3 2.0 2.3 2 0 1.4 1.3 1.1 1.1 0.7 0.4 0.1
7.6 7.0 6.5 6.2 6.1 6.8 5.4 4.0 3.2 2.0 1.6
82 78 69 63 53 45 37 25 16 10 4
5.6 5.4 5.4 5.2 5.1 5.0 5.1 4.4 2.8 2.4 1.9
44 38 35 29 25 23 17 17 8 4 1
4.8 4.4 3.9 3.1 2.3 1.6 1.2 1.2 1.2 1.1 1.1
28 25 22 20 17 15 9 7 2 0.3 0.3
SODIUM CHLORIDE
0.05
72.5 12.3 (2.3 72.4 72.3 72.3 72.2 72.4 72.2 72.2 72.2
1.3 1.3 1.0 0.9 0.6 0.4 0.3 0.1 0.2 0.1 0.0
SODIUM SULFOCYANATE'
71.6 71.2 71.3 71.6 71.5 71.7 0.8 71.5 71.7 0.7 71.8 71.5 0.6 71.6 71.9 0.5 0.4 71.7 71.8 0.3 71.8 71 9 0.2 71.9 72.1 0.1 72.1 72.2 0.05 72.3 72.2 a NaCNS is positively adsorbed. 1.0 0.9
0.4 0.3 0.2 0.2 0.3 0.3 0.1 0.1 0.2 0.1 +o. 1
Solutions That Foam at Certain Concentrations Only
The fact that solutions of some substances foam at certain concentrations only has long been known, and naturally the application of any theory of film formation to such situations has special interest, It might even be said that the peculiar behavior of such solutions affords a crucial test of the theory. These solutions, according to the theory of balanced surface
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layers, should exhibit the largest difference between static and dynamic surface tensions and the largest percentage of film. formation a t those concentrations which foam most, that is, which give the highest foam zones in the foam meter. Among common substances whose solutions exhibit the properties mentioned above, cane sugar and sulfuric acid were selected for testing the theory. They are very different in chemical character, and sulfuric acid has three concentration ranges in mhich foaming occurs with two intervening points of non-foaming. Sugar solutions are also peculiar, in that foaming occurs a t low concentrations and not at the higher ones. Unfortunately, only the two types of surface tension and the heights of the foam zones in the foam meter were determined. Observations of bubble size in the bubble zone of the foam meter showed that the percentage of film formation followed the same trend. The data are presented in Table VI. Table VI- -Relation of Static and Dynamic Surface-Tension Differences to Foam-Zone Heights in Solutions of Sulfuric Acid -SURFACE TENSION--FOAM-ZONE Static Dynamic Difference HEIGHT HzSOc Dynes/cm. Dynes/cm. Dyncs/cm. Cm. % 10 20 30 35 40 45 50
73.5 74.7 75.9 76.0 76.2 76.3 76.0 75.5 75.6 75.0 73.8 72.4 69.3 71.0 57.9
56 60 65 70 73 80 90 96
72.3 72.5 72.5 72.9 73.4 75.4 75.6 74.8 73.5 72.6 72.4 72.3 72.1 66.3 63.1
1.2 2.2 3.4 3.1 2.8 0.9 0.4 0.7 2.1 2.4 1.4 0.1 -2.8 4.7 -5.2
2.4 4.8 3.8 2.2 1.8 1.1 2.0 3.1 4.2 3.9 2.2 1.2 3.6 6.2 6.0
The results of Table VI are presented graphically in Figure 6, and it is seen that the correlation between the static and dynamic surface-tension differences and the foam-zone heights leaves nothing to be desired. Attention should, however, be called to the next to the last static value. It is out of line and rather obviously wrong, because values taken from published tables fall on the dotted continuation of the static values. It is also interesting to note that the sign of the surface adsorption of sulfuric acid solutions changes a t about 75 per cent. Table VII-Comparison of Static and Dynamic Surface-Tension Differences with Foam-Zone Heights in Solutions of Cane Sugar SURFACE TENSION FOAM-ZONE SUGAR Static Dynamic HEIGHT
% by weight 5
10 15 20 25 30 35 40
Dynes/cm. 72.4 72.8 73.1 73.3 73.6 73.6 73.9 74.3
Dyncs/cm. 72.3 72.9 73.0 73.1 73.2 73.5 73.9 74.4
Cm. 0.6 0.6
0.9 1.4 3.8 2.4 1.2 0.7
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NaCl +NaCNS Na2SOI NaCNS MgSO4 NaCNS Ethanol Na2S04 Na2SOI Soap NasSO4 Castor oil
+++ ++
It does not follow, of course, that the explanation just offered is necessarily the true one, because there might be specific effects or there might be effects due to the displacement of one substance from a surface by another one. The case for corollary (A) will, however, be bettered if it can be shown that non-foaming solutions of such paired substances exhibit no material difference between static and dynamic surface tension, and also if there is some direct relation between the static and dynamic surface-tension differencesof the solutions and the relative concentrations of these solu%FILM FORMllTlON tions to produce the non-foaming mixt u r e s . For example, t o s t o p t h e HEIGHT foaming of a given volume of a solution DlFFeRENC BETWEEN of some positively DYNAMIC adsorbed substance, ISURFACE- I I II I I -I it should require less sodium sulfate than sodium chloride Figure 5-Static and Dynamic Surface(Table V). The re- Tension Differences Foam-Zone Heights. and Percentage FilA Formation of Molar sults in Table VI11 Solutions of Six Salts (See Table IV.) show t h i s t o be true. A cut-and-try experiment was made to determine the composition of the non-foaming mixtures of sodium sulfocyanate with sodium chloride and with sodium sulfate, and also a mixture of the sulfocyanate and chloride of potassium. The results are shown in Table VIII.
1
Table VIII-Data on Non-Foaming Mixtures of Molar Solutions of Positively and Negatively Adsorbed Salts -UMACE TENSIONFOAM FiLm M I X T C R E ~ Static Dynamic Difference HEIGHTFORMATION Dynes/cm. Dynes/cm. Dynes/cm. Cm. % 100 cc. NaCNS 72,2 72,6 0.4 1.6 0.8 71 cc. NaCl
]
100 cc. NaCNS 3 2 . 5 ~ Na2SOI ~ .
loo KCNS 7 2 . 5 cc' cc. KC1 a
] ]
72.4
72.5
0.1
1.6
0.6
72.3
72.1
0.2
1.5
0.3
NaCNS and KCNS are positively adsorbed.
The correspondence between static and dynamic surfacetension differences and foam-zone heights in cane-sugar solutions is good. The results are presented graphically in Figure 7. Cane sugar, contrary to most organic substances, is slightly negatively adsorbed.
Table VI11 shows that, so far a t least as effect on surface tension, foaming, and percentage film formation goes, negative adsorption cancels positive adsorption; that is, nonfoaming mixtures have about the same concentration in the surface layer as in the mass of the solution. These experiments also show that the effects in question are determined by the ratio of surface to mass concentration and are independent of the nature of the solute.
Mixtures of Positively and Negatively Adsorbed Substances
Conclusion
Corollary A from the theory states that in the case of solutions containing both positively and negatively adsorbed substances, there should be certain mixtures that do not foam because positive and negative adsorption cancel each other by producing equality of concentration between surface and 'mass. That such non-foaming mixtures exist is easily demonstrated experimentally. The authors have shown it to be true, for example, of the following pairs:
The authors are aware of many, if not all, the experimental lines that ought to be followed in addition to those that have been presented. No attempt, for example, has as yet been made to study the relative effects of electrolytes and nonelectrolytes. Also, no consideration has been given to what might be called comparable states in respect to this question of film formation. To take a case in point, reference to Table V shows that, at equal molar concentrations, sodium
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A n interesting point in chemical philosophy should also be raised-namely, does this theory of film formation preclude all others? May there not be some other property that predominates to such an extent in some few substances that it determines the film formation? Such a property would be an unusual toughness of film accompanying a high degree of positive adsorption. Saponin and perhaps soap may belong in such a class. Saponin, for example, has been studied in connection with other work in boiler-water foaming and i t seems a t times to exhibit anomalous behavior. The effectiveness of castor oil in destroying or inhibiting foam seems also to be out of proportion to the concentration used. Literature Cited
! Figure '-static and Dynamic SurfaceFigure 6-Static and Dynamic SurfaceCane-Sugar Differences Solutions and Foam-Zone Heights Tension Differences and Foam-Zone Heights of Tension of Sulfuric Acid Solutions
sulfate shows a much higher percentage of film formation than sodium chloride. If, however, the comparison be made on the basis of equal concentration of sodium, it will be seen that the behavior of the two salts is almost identical, but the same concentration of sodium in the sulfocyanate does not give the same results. I n this case, however, the surface adsorption has the opposite sign.
(1) Foulk, IND. ENG.CHEM.,21, 815 (1929). (2) Foulk, I b i d . , 16, 1121 (1924) (3) Foulk Trans. Am. Sac. Mech. Ens., 50, FSP-5046,219 (1928). (4) Hansley, Doctor's Dissertation, Dept. Chem., Ohio State Univ., 1928. ( 5 ) Harkins, J . A m . Chem. s8*228 (1916). (6) Iredale, Phi'. Mas.3 46, (1923)' (7) McBain and Davies, J . A m . Chem. Soc., 49, 2230 (1927) ( 8 ) McBain and DuBois, I b i d , , 61, 3434 (1929). (9) Schmidt, z.physik. Chem., 39, 1108 (1912). (10) Stocker, Ibid., 94, 149 (1920). (11) Talmud and Suchowolskaja, I b i d . , 164,277 (1931).
Removal of Traces of Iron from Aqueous Solutions of Sulfates' T. W. Richmond and F. K. Cameron UNIVERSITY OF NORTH CAROLINA, CHAPELHILL, N. C.
HERE are a number of possible raw materials which might become available for the commercial production of sulfates of aluminum, sodium, magnesium, etc., if there were also available a practical way of removing from the salts small quantities of iron which contaminate the final product. Various attempts to solve this problem may be cited profitably. Zabicki (9) reports that, after two crystallizations of ammonia alum, an 86 per cent yield was obtained, containing but 0.002 per cent ferric oxide. Cooling and stirring were so controlled as to form very small crystals only. He describes an electrolytic method where a high overvoltage with hydrogen on a mercury cathode causes immediate solution of iron in the mercury, until the aqueous solution contains but 0.003 gram ferric oxide t o 100 grams aluminum oxide. Hultman and Linblad ( 4 ) propose adding to a neutral solution of aluminum sulfate, the hydroxide, carbonate, or sulfide of an alkali metal or ammonia, in quantity just sufficient to precipitate a semi-insoluble (?) basic aluminum sulfate. The liquid residue, heated under pressure, yields a precipitate, which has a composition of approximately the formula A120a.1.5S03, and which is very low in iron. Exactly opposite in principle is a proposal ( 2 ) to dissolve the raw material containing aluminum and iron in a large excess
T
1 Received
June 6, 1931.
of hot concentrated sulfuric acid. On cooling, an aluminum sulfate is obtained in this way, containing about 0.03 per cent iron. A complete removal of the iron is obtained by redissolving in hot sulfuric acid, 35-45' Baum6, and cooling, when pure aluminum sulfate crystallizes from the solution. Vittorf (8), investigating the possibility of commercially separating ferric sulfate and aluminum sulfate by dissolving in 90 per cent alcohol, reports that, on a first crystallization, the product contains about 0.1 per cent iron; but, on three recrystallizations, a 97 per cent recovery of aluminum sulfate is effected, containing 0.005 per cent iron. Adsorption methods have been tried. Thus, Fodor and Rosenberg (1) report that kaolin adsorbs ferric hydroxide from a solution of ferric chloride, but not completely, since the mineral does not neutralize the acid. Talc, however, is reported to neutralize the acid; hence the absorption of iron from the solution is complete. The authors have tried, but failed, to confirm these latter observations. Another proposal ( 7 ) is to treat solutions of aluminum sulfate containing iron, preferably when heated, with kaolin, clay, or bauxite. Part of the iron is removed by the mineral. More is removed by a little precipitated aluminum hydroxide, and the last traces by adding also some ferrocyanide, the blue prey cipitate being adsorbed on the aluminum oxide. Besides adsorption, base exchange suggests itself as a pos-
